1. Technical Field
The present invention is directed to water purification. Specifically, the present invention is directed to a composite, an electrode comprising the composite, an apparatus comprising the electrode and a method for electrochemical removal of phosphorus.
2. Description of Related Art
The regulations to the phosphorus content of discharged wastewater are getting stricter in many countries. Also, in consideration of the benefits of the resources reservation, the technique developments of removal and recovery of phosphorus have drawn much attention in the worldwide.
The concept involving wastewater treatment as well as resource recovery has been popularized in recent years. Before the wastewater is discharged, the phosphorus needs to be removed to meet the regulations. If the phosphorus can be recovered simultaneously, the treatment process will be more profitable. Nearly all phosphorus existing in the wastewater is in the form of phosphate such as organophosphate, polyphosphate and orthophosphate. The phosphorus concentration is generally low, thereby influencing the removal efficiency of phosphorus. Further, the wastewater contains many kinds of salts at the same time such as sulfate, carbonate and nitrate. The economic benefits of the recovery of phosphorus mainly depend on the target content in the raw water, the selectivity to phosphorus and the cost of the recovery technique. Therefore, increasing the phosphorus selectivity of the technique is useful to enhance the benefits of the recycling of phosphorus.
The current techniques for removal of phosphorus are mainly chemical precipitation method, biological treatment process and adsorption method. Among these techniques, the chemical precipitation method uses a variety of cations to effectively precipitate and remove phosphorus in the wastewater by adding chemical agents such as magnesium chloride, calcium chloride or ammonium bicarbonate precipitant. However, this method has poor selectivity to phosphorus, such that they are usually deposed as waste residue for landfill. The phosphorus can be recovered only when the complicated purification process is applied. In addition, the performance of chemical precipitation method is affected by the pH value, the chemical adding position and the addition dosage. Also, such method requires large quantity of chemical agents and needs the storage space and equipment, and it would produce a lot of sludge. Thus, the chemical precipitation method frequently causes the increased cost of the factory construction and maintenance.
As to the conventional biological treatment process, the content of phosphorus taken by the microorganisms is limited. Further, the microorganisms in such system are easily affected by the quality of the wastewater. Thus, the operation of the biological treatment process is unstable.
The adsorption method removes and recovers phosphorus by using the adsorbent to perform adsorption and ion exchange. The examples of the known adsorbents are silica gel, activated carbon, zeolite, molecular sieve, clay, ion-exchange resin and so on. The adsorbents need to be further modified with the different functional groups towards the specific cation or anion removal. However, the selectivity of the adsorbents is usually low, such that the adsorbents easily adsorb impurity. Further, the frequent regeneration of adsorbents consumes large amounts of acid and base reagents. Generally, the consumption ratio of regenerant and resin is about 1.5 to 5 and therefore this method is not environmentally friendly. Moreover, the common adsorbents also have the disadvantages that the specific surface area of the adsorbents is low, the adsorption rate is low (the reaction reached adsorption equilibrium after 24 to 72 hours) and it is difficult to separate the suspended powder adsorbents.